Ion implantation is a process wherein ions of a particular element are implanted into a target object. The essence of the process is the acceleration of ions to a certain energy level and the imposition of a target object into the path of the accelerated ions. The energy to which the particles are accelerated correlates with the depth to which the ions are implanted into the material. At low energy levels, the result of this process is a deposition of material onto the surface of the target object. At higher energy levels, a layer of the implanted material can be formed a given distance into the target object. If the energy levels are well controlled, the layer of implanted ions will be planar and uniform.
Ion implantation is used often in the field of semiconductor fabrication. Ions such as boron, phosphorous or arsenic are generally created from a gas source and accelerated into a silicon wafer to dope the silicon to be either p-type or n-type. Ion implantation is also used in semiconductor device fabrication in the separation by implantation of oxygen (SIMOX) process wherein the implantation of oxygen into a silicon wafer is followed by a high temperature annealing process to form a buried layer of silicon dioxide within the wafer. Another common process involves the implantation of hydrogen ions into a donor semiconductor wafer. This process was developed in the early 1990s. After the ions are implanted, the layer of silicon above the implanted ions is removed from the top of the donor semiconductor wafer. This process has been successfully utilized in the production of thin films for silicon on insulator (SOI) devices. The process generally uses hydrogen ions because their light weight allows for relatively low energy implantation.
A standard ion implanter 100 can be described with reference to FIG. 1. An ion source 101 produces ions of a particular species which then move towards a mass selection magnet 102 and an accelerator tube 103 for ultimate delivery to a target material structure 106. Ion source 101 is generally a bottle of gas 104 that provides gas to ion source structure 105 for ionization. Since the speed and species of the ions striking target material structure 106 must be well controlled, it is necessary to screen out different species of ions from ion beam 107. Mass selection magnet 102 is therefore tuned to screen out unwanted ions from ion beam 107 to prevent them from striking target material 106. Since particles with different masses bend to a different degree in a magnetic field, mass selection magnet 102 bends the desired particles just enough that they pass through filter 108. However, particles with different masses either bend too much or too little in the magnetic field and strike beam dump plates 109.
The acceleration of charged particles can result in the production of harmful radiation. This is because the collision of two charged particles can result in a nuclear reaction which in turn may result in the release of dangerous secondary particles or radiation. Collisions that produce free neutrons are particularly hazardous in that neutrons have high kinetic energy and pass through most materials but are interactive enough to cause biological damage. Given the danger of neutron radiation, the acceleration of deuterium is considered hazardous. Deuterium (2H) is a stable isotope of hydrogen having a single neutron and proton. The acceleration of deuterium can result in a deuterium-deuterium collision resulting in the generation of a neutron through the reaction 2H(d,n)3He. This reaction can occur at energies lower than 20 kilo-electron volts (keV) and will occur with increasing frequency at higher energies. At higher energies, the acceleration of deuterium can lead to a deuterium-carbon collision and the generation of a neutron through either the reaction 13C(d,n)14N or 12C(d,n)13N. The carbon in these reactions is present in ion implanters as part of mass selection magnet 102, plates 109, and various other parts such as faraday cups and the housing for target material structure 106. The 12C(d,n)13N reaction has a threshold energy of 328 keV.